Drilling fluids are reputed as the blood for bore wells. Good drilling fluid techniques are one of the important guarantees for safe, high-quality, efficient, and quick well drilling production. Drilling fluids are required in the development of oil fields.
Effective reservoir protection is an effective guarantee for improving the rate of oil and gas recovery, so that the skin factor can be an optimal negative value. If the drilling fluid is designed or used inappropriately in the well drilling process, the liquid and solid in the drilling fluid may intrude into the oil reservoir and have physical and chemical reactions with clay and other minerals in the oil and gas layer. Consequently, the permeability in the oil layer in the immediate vicinity of the well may be decreased severely, and the resistance against air and gas flow towards the bottom of the well may be increased, resulting in reduced oil yield. The injuries to the reservoir have critical impacts on low-permeability reservoirs and unnegligible damages to high-permeability reservoirs.
Usually, the permeability of high/ultra-high permeability reservoirs is 500 mD (milli Darcy) or above. For high permeability reservoirs, the permeability can be 500-2,000 mD; for ultra-high permeability reservoirs, the permeability can be 2,000 mD or above. For high/ultra-high permeability reservoirs, since the permeability is high, the solid particles in the drilling fluid may enter into the pores and pore throats in the reservoir more easily, resulting in severely decreased permeability; in addition, the drilling fluid may intrude into the reservoir more deeply owing to the high permeability, resulting in damages such as clay expanding, dispersion and migration, and water blocking, and having impacts on the accuracy of well logging; especially, it will be more difficult to drain off the drilling fluid if the foreign fluid in the drilling fluid has physical and chemical reactions with the reservoir fluid or reservoir rock; consequently, the oil and gas resources in the reservoir may be contaminated.
In the drilling process, the drilling fluid may intrude the high/ultra-high permeability reservoir easily and thereby cause damages to the reservoirs and it is difficult to recover from such damages. Such damages to the reservoir have drawn attention widely in the petroleum industry in worldwide. As the exploration is developed further, the problem becomes more and more prominent.
It is proven in field tests that the above problem can't be solved by conventional drilling fluid systems. Hence, it is especially important to develop new drilling fluid additives and reservoir protective drilling fluid techniques to meet the current challenge of complex geological conditions. Up to now, though long-time researches on reservoir protective drilling fluid techniques have been made in worldwide, and techniques such as shielded temporary plugging technique, fractal geometry-based temporary plugging technique, D90 ideal temporary plugging technique, broad-spectrum temporary plugging technique, Alkali soluble micrometer-level cellulose temporary plugging technique, D50 temporary plugging technique, filming technique, oil film technique, etc., have been developed, the protective effect of these techniques is not ideal for high/ultra-high permeability reservoirs. Hence, it is urgent task to develop a novel drilling fluid system that has enhanced plugging performance and is suitable for high/ultra-high permeability reservoirs.
The existing drilling fluids have properties that can essentially meet the requirement for cuttings carrying and well wall stability, but don't provide an ideal reservoir protection effect, and the core permeability recovery value of the reservoir is low. Though temporary plugging agents are added in these drilling fluids, the plugging performance is not high because the structural characteristics of high/ultra-high permeability reservoirs are not taken into consideration fully. Consequently, the filtrate may intrude into the reservoir and thereby results in damages such as water sensitivity and water blocking, etc. Therefore, a novel drilling fluid system that can protect high/ultra-high permeability reservoirs should be developed. The reservoir protection problem in the drilling progress of high/ultra-high permeability reservoirs has been a focus in the oil exploration and development work.